An event during a sexual encounter when one participant focuses entirely on stimulating the other participant(s) with little to no regard for his/her own sexual gratification. Generally, the pleasured individual(s) in an orgasm harvest have no idea what is about to occur, much like its precursory term, organ harvest.

He was so happy to have sex with such a beautiful woman, he performed an orgasm harvest on her.

Physiologically, it is easier for a man to harvest multiple orgasms from a woman because men usually need to rest between orgasms.
One theory behind why a person would be an orgasm harvester is low self-esteem, much like why some women become sluts.
One theory explaining why some men in particular would be an orgasm harvester is if a typical nice guy gets stuck in the friend zone with every woman he meets, when he finally does get to have sex, he pleasures the hell out of her with the hopes he can do it again in the future.

Jack: You missed one hell of a party. Throughout the night, Jill kept sneaking off with random dudes and sucking them off. She must’ve pleasured half the guys there!
Jim: Wow, talk about your orgasm harvester.

Candy: Lee seems like a decent enough guy, but he’s probably nothing special in the sack.
Sarah: I disagree, my friend hooked up with him and it turns out he’s an orgasm harvester.
Candy: That's hot.